_B._ The same compounded motion that heats, namely, that of fire, if it be strong enough. For all motion compounded is an endeavour to dissipate, as I have said before, the parts of the body to be moved by it. If therefore hardness consist only in the pressing contact of the least parts, this motion will make the same parts slide off from one another, and the whole to take such a figure as the weight of the parts shall dispose them to, as in lead, iron, gold, and other things melted with heat. But if the small parts have such figures as they cannot exactly touch, but must leave spaces between them filled with air or other fluids, then this motion of the fire, will dissipate those parts some one way, some another, the hard part still hard; as in the burning of wood or stone into ashes or lime. For this motion is that which maketh fermentation, scattering dissimilar parts, and congregating similar. _A._ Why do some hard bodies resist breaking more one way than another? _B._ The bodies that do so, are for the most part wood, and receive that quality from their generation. For the heat of the sun in the spring-time draweth up the moisture at the root, and together with it the small parts of the earth, and twisteth it into a small twig, by its motion upwards, to some length, but to very little other dimensions, and so leaves it to dry till the spring following; and then does the same to that, and to every small part round about it; so that upward the strength is doubled, and the next year trebled, &c. And these are called the grain of the wood, and but touch one another, like sticks with little or no binding, and therefore can hardly be broken across the grain, but easily all-along it. Also some other hard bodies have this quality of being more fragile one way than another, as we see in quarrels of a glass window, that are aptest many times to break in some crooked line. The cause of this may be, that when the glass, hot from the furnace, is poured out upon a plain, any small stones in or under it will break the stream of it into divers lines, and not only weaken it, but also cause it falsely to represent the object you look on through it. _A._ What is the cause why a bow of wood or steel, or other very hard body, being bent, but not broken, will recover its former degree of straightness? _B._ I have told you already, how the smallest parts of a hard body have every one, by the generation of hardness, a circular, or other compounded motion; such motion is that of the smallest parts of the bow. Which circles in the bending you press into narrower figures, as a circle into an ellipsis, and an ellipsis into a narrower but longer ellipsis with violence; which turns their natural motion against the outward parts of the bow so bent, and is an endeavour to stretch the bow into its former posture. Therefore if the impediment be removed, the bow must needs recover its former figure.